The integrin mediated adhesive interactions of cells with other cells and between cells and the extracellular matrix are believed to play critical roles in a wide variety of processes including, for example, modulation of the immune system, regulation of developmental processes and tumor progression and metastasis. These molecules also transduce information from the extracellular to the intracellular environment through poorly understood signalling mechanisms. The integrins represent one of the best characterized superfamilies of adhesion receptors. Integrins are glycoprotein heterodimers which contain a non-covalently associated .alpha. and .beta. subunit. Integrin subunits are transmembrane proteins which contain an extracellular domain for interacting with an extracellular matrix or cellular component, a transmembrane domain spanning the cell membrane and a cytoplasmic domain for interacting with one or more cytoskeletal components.
There are fourteen known .alpha. subunits and eight known .beta. subunits which can pair to form at least twenty different integrin molecules. Several distinct integrin .alpha. chains are capable of pairing with one type of .beta. chain to form a .beta. chain subfamily. Thus, for example, the .beta..sub.1 subfamily includes seven members (also known as the VLA proteins: .alpha..sup.1 .beta..sub.1 -.alpha..sup.7 .beta..sub.1); the .beta..sub.2 subfamily includes three members (the leukocyte cell adhesion molecules or LeuCAMs: .alpha..sup.L .beta..sub.2 or LFA-1, .alpha..sup.M .beta..sub.2 or Mac-1 and .alpha..sup.x .beta..sub.2 or p150,95) and the .beta..sub.3 subfamily includes two members (.alpha..sup.v .beta..sub.3, .alpha..sup.IIb .beta..sub.3). In some instances, an .alpha. chain may pair with more than one .beta. chain, e.g., .alpha..sup.4 can pair with .beta..sub.1 or .beta..sub.7.
The integrin .alpha. chains have in common a seven-fold repeated amino acid motif, of which the last three or four motifs include divalent cation binding sites. All known .alpha. chains have been divided into one of two structural groups on the basis of amino acid sequence homology and the presence or absence of two structural features (described below).
The first group of .alpha. chains contains a proteolytic cleavage site located in the extracellular domain, proximal to the transmembrane region. Post-translational cleavage of the .alpha. chain precursor yields two fragments which (with one exception) remain associated by a disulfide linkage. The smaller fragment includes a short portion of the extracellular domain, the transmembrane and the cytoplasmic domains. The larger fragment contains the major portion of the .alpha. chain extracellular domain. This group of post-translationally cleaved integrin .alpha. subunits includes .alpha..sup.3, .alpha..sup.4, .alpha..sup.5, .alpha..sup.6 (formerly called .alpha..sup.E), .alpha..sup.v and .alpha..sup.IIb, although the .alpha..sup.4 molecule is a more distant member of the group since it is less similar to the other cleaved integrin .alpha. subunits based upon homology analysis, is cleaved near its mid-point to yield two fragments of nearly equal size and further, because the fragments are not disulfide linked (Teixido, J. et al., (1992) J. Biol. Chem. 267, 1786-1791; Rubio, M. et al., (1992) Eur. J. Immunol. 22, 1099-1102).
Members of the second group of integrin .alpha. subunits do not include the above-described proteolytic cleavage site. Moreover, the second group of .alpha. subunits is characterized by the presence of an additional region known as the "I" (inserted) domain. Homologous I domains have been identified in complement factors B and C2, von Willebrand's factor, cartilage matrix glycoprotein and collagen type VI.
The importance of integrins with respect to modulation of the immune system is illustrated by the condition, leukocyte adhesion deficiency (LAD), a disorder that is characterized by profound immunodeficiency. Individuals afflicted with LAD are unable to express the .beta..sub.2 integrin subfamily (Hogg, N. (1989) Immunol. Today 10, 111-114). Thus, while it has been known for some time that integrins and other adhesion molecules function in immune system modulation, e.g., by playing a role in the adhesion of peripheral lymphocytes to endothelium and in homing to lymph nodes. However, relatively little is known about the molecules that function in the mucosal immune system, a subset of the general immune system which includes the lymphocytes which populate the gastrointestinal, genito-urinary and respiratory tracts, and the mammary glands. In particular, little is known about the molecules which function in mucosal lymphocyte homing. (see, Cepek, K. et al., (1993) J. Immunol. 150, 3459-3470 and references cited therein).
Recently, we described a novel integrin heterodimer that is expressed on intra-epithelial T lymphocytes (iIEL), i.e., the population of T lymphocytes located along the baso-lateral surfaces of the epithelial cells which line the mucosa, adjacent to the epithelial cell basement membrane. (Parker, C. M. et al., (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1924-1928). Originally defined by an antibody which recognizes the human mucosal lymphocyte 1 antigen (HML-1), the novel integrin is present on &gt;90% of intestinal IEL (iIEL) and on approximately 40% of lamina propria T lymphocytes (which lie between the epithelial basement membrane and the muscularis mucosae) (Cerf-Bensussan, N. et al., (1987) Eur. J. Immunol. 17, 1279-1285). The HML-1 antigen contains a novel .alpha. chain (designated .alpha..sup.E, for "epithelial associated") associated with a .beta..sub.7 chain (Parker, C. M. et al., (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1924-1928). Although the HML-1 .beta..sub.7 chain has been cloned (Yuan, Q. A. et al., (1990) Int. Immunol. 2, 1097-1108; Erle, D. J. et al., (1991) J. Biol. Chem. 266, 11009-11016), little is known about the primary structure of the .alpha..sup.E chain.
Cloning of the .alpha..sup.E chain has proven to be problematic because of the difficulty in obtaining an adequate number of intra-epithelial cells from which the .alpha..sup.E chain could be purified and sequenced. In addition, the relatively large length of the .alpha..sup.E gene has hampered cloning efforts because of the propensity to lose the 5' portion of relatively long genes during cDNA synthesis.